KAPPA CEN (Kappa Centauri). Centaurus, the Centaur, blends smoothly into the stars of Lupus, the Wolf, the two constellations filled with bright, prominent stars. The two are commented through mythology, as in one story Centaurus sacrifices Lupus upon Ara, the Altar. The main stellar connection between them is made by notable third magnitude (2.97) Kappa Centauri and, about a degree to the south, somewhat brighter Beta Lupi. Though perhaps looking paired, they are not, Beta Lup more than 500 light years away, Kappa Cen much closer at 383 light years (give or take 28). Kappa Cen is, however, a member of the Upper Centaurus- Lupus association of hot stars that is centered at a distance of 460 light years, showing how spread out these associations are. Beta Lupi could be a member as well, thus reconnecting them. A close look, then a very close look, reveals more relations, that Kappa Cen is actually a triple star. Visually, the bright star, Kappa Cen A, a hot blue class B (B2) subgiant, is paired with 11th magnitude Kappa B. Lying four seconds of arc away, from its brightness Kappa B is a class K2 dwarf. Kappa Cen A is then split with sophisticated instrumentation into two just a tenth of a second of arc apart, and is made of the hot B star (now Kappa Cen Aa, of magnitude 3.4), and Kappa Ab, a 4.7 magnitude star that is probably a class A0 dwarf. Aa and Ab are difficult to separate, to say the least. With respective temperatures of 19,845 Kelvin, and (from the class) 9500 K, after about a 15 percent correction for dimming by interstellar dust, Aa and Ab have luminosities somewhere around 3500 and 190 times that of the Sun, which lead to radii of 5.0 and 5.1 solar. With a slow projected equatorial rotation speed of 20 kilometers per second, the rotation period of Aa is under 13 days (the rotation probably more or less pole-on). Theory then allows masses of 7.5 and 3.0 Suns. It also shows Aa to be not a subgiant (a star that has given up core hydrogen fusion, or is about to), but a dwarf about two-thirds of the way through its lifetime of 33 million years, such divergences in classification not uncommon. However, Ab then seems to be a subgiant, which cannot be, as lower mass dwarfs take longer to burn out than do those of higher mass. Moreover, Aa should be larger than Ab, not smaller, both of these anomalies showing that one or more of the input parameters is wrong. We can probably rely on those of Aa more than those of Ab. From distance and separation, Ab is separated from Aa by about a dozen Astronomical Units, and assuming the masses to be correct, take a dozen years to orbit each other. Out in the distance, Kappa Cen B lies at least 470 Astronomical Units away from the inner pair. Adopting 0.75 solar for Kappa Cen B, the little class K star must take at least 3000 years to orbit the inner pair. From its perspective, Aa and Ab would get as far as a bit over a degree apart. Not big enough to explode as a supernova, Kappa Cen Aa will die as a fairly massive white dwarf rather like Sirius B.
Written by Jim Kaler 6/29/12. Return to STARS.